MFJ-201 MFJ MFJ201[MFJ201]

Back in the vacuum tube days they called these babies grid dip meters and every ham and experimenter either had one or wanted one. The MFJ-201 solid state (hence no "grid") is smaller, lighter, and vastly more versatile than its ancestor.
Use it as a traditional dip meter to fina a coil's resonant frequency.

Couple it to an antenna, or to coax lines to get their electrical length.

Or use it as an absorption wavemeter for measureing transmitter output, neutralizing tank circuits, or a field strength meter.